The disclosed invention relates in general to the comparison of an optical frequency to a radio frequency and more particularly to a technique that enables a rational ratio N/M to be established between an optical frequency and a radio frequency (RF). In one technique presented in the article Z. Bay, et al. Measurement of an Optical Frequency and the Speed of Light, Phys. Rev. Let., vol. 29, No. 3, 17 July, 1972, p.189, a laser beam of frequency f.sub.1 is modulated at radio frequency f.sub.2 before passing through a Fabry-Perot interferometer cavity. The length L of the cavity and f.sub.2 are adjusted so that the maximum intensities of both sidebands (i.e., at f.sub.1 +f.sub.2 and f.sub.1 -f.sub.2) pass through the cavity. A first etalon prevents the carrier frequency f.sub.1 from entering the interferometer cavity and a pair of tuned etalons separately pass each sideband after the laser beam passes through the interferometer cavity. f.sub.1 is adjusted in response to the difference in the two sideband signals and L is adjusted in response to the sum of the two sideband signals.
Unfortunately, this technique is not suitable for use over a broad spectral range. First, to avoid unwanted feedback that can produce a shift in the laser frequency or even stop it from lasing, an isolator containing a quarter wave plate is included in the laser beam path between the laser and the interferometer. Because a quarter wave plate cannot produce exactly one quarter wave retardation over a wide laser frequency range, this technique is not suitable over a broad spectral range. Second, the etalons would need to be gang tuned to the laser wavelength. This tuning would itself require some kind of moderate accuracy wavemeter, thereby limiting the spectral range of practical utility.